The invention relates to a method of rendering a microporous membrane hydrophilic. More specifically, the invention relates to the addition of a specified agent to the solvent system used in casting such membranes to enhance the permanent hydrophilicity of the membrane. The invention further relates to the membrane produced using the subject casting composition, including the solvent system additive.
Many synthetic polymeric membranes are made by known phase inversion processing. Generally, the polymeric matrix material used for membranes prepared for micro- and ultra-filtration is hydrophobic in nature, thus making the filtration of aqueous systems difficult. To render the hydrophobic membranes hydrophilic, and thereby usable for the filtration of aqueous solutions, a wetting agent can be added to the polymeric system used to cast the membrane. The wetting agent, which is a surface active agent, can be applied to the membrane matrix in a post-production treatment step. Wetting agents added in this manner, however, tend to suffer from extensive leaching problems, introducing foreign materials, such as wetting agents, into the filtrate. The leaching problem can be addressed by using a polymeric wetting agent, crosslinked during the post-treatment step onto the membrane matrix (e.g., U.S. Pat. No. 5,376,274). The addition of a post-treatment step, however, complicates the membrane production process.
The leaching problem encountered when using post-treated membranes has also been addressed by a process wherein the wetting agent is trapped within and on the membrane surface. This is generally accomplished by incorporation of the wetting agent into the original membrane casting formulation, resulting in a portion of the agent being trapped at the membrane surface, as is disclosed in U.S. Pat. No. 4,900,449. Some factors which affect the amount of agent which is trapped at the membrane surface include the speed with which the precipitation is accomplished, and the extent of blending of the various components. In the '449 patent, hydrophilicity is accomplished by using hydrophilic additives such as polyethylene glycol and polyvinylpyrrolidone. This membrane system nonetheless suffers from a certain amount of leaching of the additive from the membrane during the washing step, most likely due to incompatibility of the hydrophilic polyethylene glycol and polyvinylpyrrolidone additives with the hydrophobic polyethersulfone membrane matrix polymer. This membrane is thus left with an uncontrolled or inconsistent wetting agent content, and further exhibits a lack of uniformity across the membrane surface.
Another attempt at solving the leaching problem encountered with hydrophobic membranes rendered hydrophilic by the addition of a wetting agent involved the use of a block copolymer as a matrix additive, the copolymer comprising a combination of hydrophobic and hydrophilic portions. The hydrophobic portion of the copolymer additive, being more compatible with the hydrophobic matrix polymer, tends to tangle with and thus become anchored to the membrane matrix polymer, leaving the hydrophilic portion of the copolymer at the membrane surface. The interaction between the main hydrophobic matrix membrane, the block copolymer additive and the solvent system used for the formulation is a complicated one. Variations in the type and amount of the various components of the system as a whole may cause more or less entanglement of the block copolymer with the membrane matrix and thereby affect the hydrophilicity of the membrane. While the addition of a block copolymer wetting agent improves over other prior art methods noted above, it nonetheless suffers, even slightly, from problems related to the amount of agent actually retained at the membrane surface.
It has remained for the current invention to provide a means by which a block copolymer, added to the matrix polymer material to render the polymeric membrane cast therefrom hydrophilic for use in aqueous separation processes, can be substantially permanently anchored at the membrane surface. This avoids much of the leaching problem during use, as well as avoiding problems related to variations in membrane surface hydrophilicity.